Catalyst carrier and fuel cell using the same

ABSTRACT

A catalyst carrier, being characterized in that a catalyst metal for promoting an oxidation-reduction reaction is carried on a vapor-grown carbon fiber having an average outer diameter of from 2 nm to 500 nm, which has been subjected to a crushing treatment so as to have a BET specific surface area of from 4 m 2 /g to 100 m 2 /g and an aspect ratio of from 1 to 200, and exhibiting high activity per unit amount of a catalyst metal, a low reaction resistance and an improved output density, and is useful for a fuel cell; a production method thereof and a fuel cell using the catalyst carrier.

CROSS REFERENCE TO THE RELATED APPLICATIONS

This application is a Rule 53(b) Continuation of U.S. application Ser.No. 11/041,972 filed Jan. 26, 2005 which claims benefit of JapanesePatent Application No. 2004-018879 filed Jan. 27, 2004 and U.S.Provisional Application No. 60/541,505 filed Feb. 4, 2004. Theabove-noted applications are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The present invention relates to a catalyst carrier. More particularly,the present invention relates to a catalyst carrier, which can be usedas an electrode catalyst of a fuel cell, wherein a catalyst metal iscarried on a carbon fiber; a production method for the catalyst carrier;and a fuel cell using the catalyst carrier.

BACKGROUND ART

A solid polymer type fuel cell is attracting attention to be used for acell automobile and a portable power supply since it is compact and canobtain a high current density when operated at room temperature comparedwith a phosphoric-acid type fuel cell and a molten carbonate type fuelcell. Further, many proposals on components, system compositions and thelike in such fields have been made. A stack structure of a conventionalsolid polymer type fuel cell is a sandwich structure of, for example, ofseparator/electrode (oxygen electrode)/electrolyte membrane/electrode(hydrogen electrode)/separator. Required characteristics of an electrodefor this fuel cell are to prevent the electrode from poisoning by carbonmonoxide and to enhance activity per unit amount of a catalyst metal.For the purpose of preventing such poisoning and enhancing the activity,many trials have been made to date on metals or alloys thereof to beused as catalysts as described in JP-A-2001-85020 (U.S. Pat. No.6,689,505), which describes that a particle size of a catalyst ispreferably several nm.

On the other hand, as for carbon to be used for a carrier, particulatecarbon such as ordinary carbon black is used as described inJP-A-8-117598, JP-A-2003-201417 (EP 1309024) and JP-A-2001-357857.However, since the contact between carbon particles is conducted by apoint contact, there is a problem that resistance is large and gaspermeability is insufficient. In order to solve these problems, it hasbeen considered effective to change the particulate carbon to fibercarbon to be used for the carrier as described in JP-A-7-262997,JP-A-2003-317742 and JP-A-2003-200052.

As for carbon fibers, a vapor-grown carbon fiber, a carbon nanotube anda PAN type carbon fiber are known. However, in any of reports which havebeen made public to date, a technique to produce an electrode comprisinga carbon fiber on which fine catalyst particles are uniformly carriedwith a high density has not been described.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a vapor-grown carbonfiber capable of enhancing an activity per unit amount of a catalystmetal, reducing a reaction resistance and enhancing an output densityand appropriate as a catalyst carrier and the like, a catalyst carriercarrying a metal catalyst, production methods thereof and an applicationthereof for a fuel cell.

The present invention provides a catalyst carrier, a production methodand an application thereof as follows:

1. A catalyst carrier, being characterized in that a catalyst metal forpromoting an oxidation-reduction reaction is carried on a vapor-growncarbon fiber having an average outer diameter of from 2 nm to 500 nm,which has been subjected to a crushing treatment so as to have a BETspecific surface area of from 4 m²/g to 100 m²/g and an aspect ratio offrom 1 to 200.

2. The catalyst carrier as described in 1 above, wherein a ratio of anaverage fiber length after the crushing treatment to an average fiberlength before the crushing treatment is 0.8 or less.

3. The catalyst carrier as described in 1 or 2 above, wherein a ratio ofa specific surface area after the crushing treatment to a specificsurface area before the crushing treatment is 1.1 or more.

4. The catalyst carrier as described in any of 1 to 3 above, wherein thevapor-grown carbon fiber is a carbon fiber comprising a branchedvapor-grown carbon fiber;

5. The catalyst carrier as described in any of 1 to 4 above, wherein thecatalyst metal is at least one metal selected from the group consistingof platinum and transition metals belonging to the groups IV and V inthe periodic table or any one of alloys thereof.

6. A production method for a catalyst carrier, being characterized inthat a catalyst metal for promoting an oxidation-reduction is supportedon a vapor-grown carbon fiber having an average outer diameter of from 2nm to 500 nm, which has been obtained by thermally decomposinghydrocarbon, or a vapor-grown carbon fiber obtained by performing athermal treatment of the thus-obtained vapor-grown carbon fiber at atemperature of from 600° C. to 1300° C. in an atmosphere of an inertgas, which have been crushed so as to have a BET specific surface areaof from 4 m²/g to 100 m²/g and an aspect ratio of from 1 to 200.

7. The production method for the catalyst carrier as described in 6above, wherein support of the catalyst metal is performed by a liquidphase reduction method.

8. The production method for the catalyst carrier as described in 6 or 7above, wherein, after being crushed, the vapor-grown carbon fiber issubjected to a thermal treatment at a temperature of from 2000° C. to3000° C. in an atmosphere of an inert gas.

9. The production method for the catalyst carrier as described in 6 or 7above, wherein, before being crushed, the vapor-grown carbon fiber issubjected to a thermal treatment at a temperature of from 2000° C. to3000° C. in an atmosphere of an inert gas.

10. The production method for the catalyst carrier as described in anyof 6 to 9 above, wherein such crushing is performed by dry crushingusing an impact force.

11. The production method for the catalyst carrier as described in 10above, wherein crushing is performed in an atmosphere containing oxygenin a concentration of 5% by volume or more.

12. A catalyst carrier obtained by the method as described in any of 6to 11 above.

13. An electrode material, wherein a catalyst layer comprising acatalyst carrier as described in any of 1 to 5 and 12 above is formed onan electrically conductive base material.

14. A membrane electrode assembly for a fuel cell, which comprises anelectrode wherein a catalyst layer and a gas diffusion layer areprovided on both faces of an electrolyte membrane, being characterizedin that the catalyst layer comprises the electrode material as describedin 13 above.

15. A cell of a fuel cell, comprising the membrane electrode assemblyfor the fuel cell as described in 14 above which is sandwiched byseparators.

16. A fuel cell, comprising two or more of the cells for the fuelbattery as described in 15 above being laminated one on another.

Further, the present invention also relates to a vapor-grown carbonfiber as follows:

17. A vapor-grown carbon fiber, being characterized by having on itssurface a physical site capable of carrying a catalyst metal.

18. The vapor-grown carbon fiber as described in 17 above, wherein thephysical site is a defect generated by a chemical and/or physicalaction.

19. The vapor-grown carbon fiber as described in 18 above, wherein thephysical action is an action by an impact and/or shearing force.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic vertical cross-sectional diagram showing astructure in the vicinity of an end portion of a conventional finecarbon fiber;

FIG. 2 is a schematic vertical cross-sectional diagram showing astructure in the vicinity of an end portion of another conventional finecarbon fiber;

FIG. 3 is a schematic vertical cross-sectional diagram for explaining astructure in the vicinity of an end portion of a fine carbon fiber to beused in the present invention;

FIG. 4 is a schematic vertical cross-sectional diagram for explaining astructure in the vicinity of an end portion of a fine carbon fiber to beused in the present invention;

FIG. 5 is a schematic side view seen from a direction of an end portionof the fiber according to FIG. 4;

FIG. 6 is a schematic vertical cross-sectional diagram for explaining astructure in the vicinity of an end portion of a fine carbon fiber to beused in the present invention;

FIG. 7 is a schematic vertical cross-sectional diagram for explaining astructure in the vicinity of an end portion of a fine carbon fiber to beused in the present invention;

FIG. 8 is a schematic vertical cross-sectional diagram for explaining astructure in the vicinity of an end portion of a fine carbon fiber to beused in the present invention;

FIG. 9 is a schematic vertical cross-sectional diagram for explainingstructures in the vicinities of both end portions of a fine carbon fiberto be used in the present invention;

FIG. 10 is a schematic vertical cross-sectional diagram for explainingstructures in the vicinities of both end portions of a fine carbon fiberto be used in the present invention;

FIG. 11 is a transmission electron micrograph of the catalyst carrieraccording to Example 1;

FIG. 12 is a transmission electron micrograph of the catalyst carrieraccording to Example 2;

FIG. 13 is a transmission electron micrograph of the catalyst carrieraccording to Example 3;

FIG. 14 is a transmission electron micrograph of the catalyst carrieraccording to Example 4;

FIG. 15 is a transmission electron micrograph of the catalyst carrieraccording to Comparative Example 1;

FIG. 16 is a transmission electron micrograph of the catalyst carrieraccording to Comparative Example 2;

FIG. 17 is a transmission electron micrograph of the catalyst carrieraccording to Comparative Example 3; and

FIG. 18 is a graph showing Tafel plots of fuel batteries using catalystcarriers according to Examples 1 to 4 and Comparative Examples 1 to 3.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in detail.

A vapor-grown carbon fiber to be used as a carrier of a catalyst carrierof the present invention has an average outer diameter of from 2 to 500nm and has been subjected to a crushing treatment so as to have a BETspecific surface area of from 4 to 500 m²/g and an aspect ratio of from1 to 200. Preferably, the average outer diameter is from 15 to 200 nm;the BET specific surface area is from 10 to 200 m²/g; and the aspectratio is from 2 to 150. Further, preferably, an average fiber length isfrom 2 to 100 μm. By using as a carrier the vapor-grown carbon fiberwhich has been subjected to the crushing treatment so as to haveproperties within the respective ranges as described above, when acatalyst metal is carried thereon, the carrier is capable of carryingthe catalyst metal while being in a state of particles having a smalldiameter and a large specific surface area and, as a result, a catalyticactivity can be enhanced.

The crushing treatment has preferably been performed such that anaverage fiber length after the treatment becomes 0.8 or less when theaverage fiber length before the treatment is taken as 1. Further, thecrushing treatment has preferably been performed such that a BETspecific surface area after the treatment becomes 1.1 or more and, morepreferably, 1.4 or more, when the BET specific surface area before thetreatment is taken as 1.

The vapor-grown carbon fiber to be used in the present inventionpreferably contains a branched carbon fiber, and thereby an electricconductive pass of the carbon fiber is formed and, accordingly, electricconductivity as the catalyst carrier can be enhanced.

The vapor-grown carbon fiber to be used in the present invention isprepared by subjecting a fine carbon fiber produced by a vapor phasemethod to a crushing treatment. Performing the crushing treatmentenables to produce a fine carbon fiber having a multi-layer structure,which comprises a discontinuous surface of a graphene sheet having afracture surface at an end portion of the fiber; a continuous surfaceformed by combining an end portion of at least one graphene sheet withan end portion of an adjacent graphene sheet; and a hollow space in acenter axis. By such a structure as described above, electric resistanceof the carbon fiber is reduced and electric conductivity as a catalystcarrier is enhanced.

Characteristics of the fine carbon fiber are described with reference toFIGS. 1 to 10. In these figures, graphene sheets (layers of graphite orcrystals similar to graphite) are schematically shown in solid lines.

As shown in a schematic vertical cross-sectional diagram according toFIG. 1 or 2, the fine carbon fiber comprises a discontinuous surface (1)of a graphene sheet having a fracture surface or a closed surface (2)having a continuous surface of the graphene sheets at the end portion ofa fiber, and a hollow space (3).

On the other hand, as shown in a schematic cross-sectional diagramaccording to FIG. 3 or 4, a preferred mode of the fine carbon fiberaccording to the invention is such a fine carbon fiber having a hollowspace (3) produced by a vapor phase method, which comprises adiscontinuous surface (1) of a graphene sheet having a fracture surfaceat an end portion of the fiber and a continuous surface (2) formed bycombining an end portion of at least one graphene sheet and an endportion of an adjacent graphene sheet. The fracture surface denotes aplane generated by crushing or the like. Continuity of the graphenesheets is broken at the fracture surface, to thereby allow an edgecarbon atom at a fracture portion inside a basal surface, an edge carbonatom at a border portion of a crystallite or the like to appear. Thefracture surface is, for example, an end surface at approximately rightangles to a center axis of the carbon fiber. Even in the fiber having alow aspect ratio (1 to 200), the hollow space and a multi-layerstructure (growth ring structure) are maintained.

The carbon fiber according to FIG. 3 has a closed surface (2) having twocontinuous surfaces of the graphene sheets; in one portion (a), twoadjacent graphene sheets are combined with each other at respective endportions, while, in the other portion (b), among adjacent four graphenesheets, outermost two graphene sheets are combined with each other atend portions, while inner two graphene sheets are combined with eachother at end portions, respectively. The discontinuous surface (1) ofthe graphene sheet is present at the side of a hollow space (3) adjacentto the portion (a).

The carbon fiber according to FIG. 4 is a carbon fiber comprising fourlayers (4, 6, 8, 10) of graphene sheets; outer two layers of thegraphene sheets (4, 6) form continuous surfaces (2(a)) in which endportions thereof are combined with each other all over thecircumference, while inner two layers of the graphene sheets (8, 10)simultaneously form a closed portion (2(b)) having a continuous surfacein which end portions thereof are combined with each other and a portion(1(a)) in which end portions thereof have a discontinuous surface.

FIG. 5 is a schematic side view of the carbon fiber having the structureaccording to FIG. 4 seen from a direction of the end portion thereof.White portions show continuous surfaces (2(a), 2(b)), while a blackportion shows a discontinuous surface (1(a)). A center portion is ahollow portion, while a grey portion shows an interface between graphenesheets (6) and (8). A continuous surface of the graphene sheets which ispresent at an end of the fine carbon fiber is continuous also in thedirection of the circumference; however, it is considered that thediscontinuity may be generated also in the direction of thecircumference by an influence of a defect caused by crushing, a thermaltreatment temperature, an impurity component other than carbon, etc.

Each of FIGS. 6 to 8 shows a carbon fiber comprising eight layers of thegraphene sheets.

In FIG. 6, outer two layers of the graphene sheets (12, 14) form acontinuous surface in which end portions thereof are combined with eachother all over the circumference, while each of the other six layers ofthe graphene sheets form a discontinuous surface.

In FIG. 7, an outermost layer of the graphene sheet (16) and the fourthfrom the outermost layer of the graphene sheet (22), and the second andthe third layers from the outermost layer of the graphene sheet (18, 20)which are adjacent to each other are combined with each otherrespectively at the end portions to form continuous surfaces all overthe circumference, while each of the other four layers of the graphenesheets form a discontinuous surface.

In FIG. 8, an outermost layer of the graphene sheet (24) and the sixthlayer from the outermost layer of the graphene sheet (34), the secondand fifth layers from the outermost layer of the graphene sheet (26,32), and the third and fourth layers from the outermost layer of thegraphene sheet (28, 30) which are adjacent to each other are combinedwith each other respectively at the end portions to form continuoussurfaces all over the circumference, while each of the other two layersof the graphene sheets form a discontinuous surface.

Each of FIGS. 9 and 10 shows an entire picture of a fine carbon fiber.FIG. 9 shows a mode in which one end of the fiber forms only continuoussurfaces in the same manner as conventional and the other end has bothcontinuous and discontinuous surfaces, while FIG. 10 shows a mode inwhich both ends of the fiber have both continuous and discontinuoussurfaces.

The continuous surface which exists in the same surface as the fracturesurface shows a surface in which a defect is generated in the graphenesheet laminated by thermal chemical vapor deposition and, accordingly,the graphene sheet has lost the regularity and is combined with anadjacent graphene sheet; or a fracture end of a graphene sheet isrecombined with an end of another graphene sheet by a high temperaturetreatment of 2000° C. or more. A curved portion of the continuoussurface comprises one or more of graphene sheets; however, in a case inwhich the number of the laminated graphene sheets is small, namely, in acase in which a curvature radius of such curved graphene sheets issmall, the fiber is hard to stably exist since a surface energy of thecurved portion is large, and therefore, the number of the laminatedgraphene sheets at the curved portion is preferably three more, morepreferably five or more and, particularly preferably, five to ten.

The vapor-grown carbon fiber used in the present invention can beproduced by crushing a vapor-grown carbon fiber produced by a vaporphase method and, preferably, a carbon fiber comprising a branchedcarbon fiber (produced by a method as described in, for example,JP-A-2002-266170 (WO02/49412)).

The vapor-grown carbon fiber to be used in such production canordinarily be obtained by thermally decomposing an organic compoundwhile using an organic transition metal compound as a catalyst. Theorganic compound which can serve as a raw material for the carbon fiberis a compound selected from among toluene, benzene, naphthalene, gasessuch as ethylene, acetylene, ethane, a natural gas, carbon monoxide andthe like and mixtures thereof. Thereamong, aromatic hydrocarbons such astoluene and benzene are preferred. The organic transition metal compoundis an organic compound containing a transition metal which can be acatalyst, specifically, any one of metals belonging to the groups IV toX in the periodic table. Particularly, compounds such as ferrocene andnickelocene are preferred.

The carbon fiber is, preferably, a carbon fiber in which an interlayerdistance (d₀₀₂) of a hexagonal carbon layer (002) by an X-raydiffractometry is 0.345 nm or more, a ratio (Id/Ig) of a peak height(Id) of the band at 1341 to 1349 cm⁻¹ to a peak height (Ig) of the bandat 1570 to 1578 cm⁻¹ in a Raman scattering spectrum is 1 or more. Inthis case, Id denotes a broad band region corresponding to an increaseof irregularity of a carbon structure, while Ig denotes a relativelysharp band region associated with a perfect graphite structure.

Raw materials to be crushed can be subjected to a thermal treatment at600 to 1300° C. in order to remove an organic substance such as tarattached on a surface of the carbon fiber obtained by thermaldecomposition.

As for crushing methods, a rotary crusher, a high-speed mill, a ballmill, a medium stirring mill, which adopts a method of crushing thefiber using an impact force, a jet crusher and the like can be utilized.Particularly, a vibrating mill such as a circular vibrating mill, agyratory vibrating mill or a centrifugal mill is preferred. As forcrushing media, ceramics balls of alumina, zirconia, silicon nitride andthe like, or metal balls of stainless steel and the like can be used.Among these, stainless steel balls which can be removed by ahigh-temperature thermal treatment are preferred.

Further, it is advantageous to perform a dry type crushing in theabsence of water and/or an organic solvent, since it is not necessary toperform a post-treatment step such as removing a dispersant after thecrushing, drying the solvent or crushing a dry coagulated fiber.

The crushing is preferably performed in an atmosphere having an oxygenconcentration of 5% by volume or more. By allowing oxygen to be presentin a volume of 5% or more, a surface of a crushed carbon fiber ismodified, to thereby facilitate a catalyst metal to be carried. Thecrushing is preferably performed in the air.

Further, in a pretreatment or a post-treatment of the crushing, agraphitization treatment can be performed for the purpose of enhancingelectric conductivity of the vapor-grown carbon fiber. Thegraphitization treatment can be performed by carrying out a thermaltreatment at 2000 to 3000° C. in an atmosphere of an inert gas.

The catalyst carrier according to the present invention comprises acrushed vapor-grown carbon fiber and a catalyst metal for promoting anoxidation-reduction reaction carried on the carbon fiber.

Catalyst metals for promoting the oxidation-reduction reaction are atleast one element selected from the group consisting of transitionmetals belonging to the groups IV and V in the periodic table comprisingplatinum and other platinum metal elements or mixtures thereof,preferably, platinum metal elements (nickel, palladium and platinum) oran alloy containing these metal elements.

A method for carrying the catalyst metal on the crushed vapor-growncarbon fiber is not particularly limited and is performed by, forexample, a liquid phase reduction method. An example in which fineplatinum particles are carried on the crushed vapor-grown carbon fiberby the liquid reduction method is described below.

Firstly, the crushed carbon fiber is dispersed in a distiled water and apH value of the resultant dispersion is adjusted by using, for example,sodium carbonate. A dispersion operation can be performed by, forexample, an ultrasonic wave treatment while confirming a dispersioncondition by, for example, visual observation. Since the vapor-growncarbon fiber is high in hydrophobicity, it is preferable to enhancehydrophilicity by performing a surface treatment (hereinafter, referredto also as “hydrophilization treatment”) on the carbon fiber in advanceand, by this treatment, a specific surface area of the catalyst metal tobe carried can be improved. The surface treatment can be performed in,for example, an acid solution (such as an aqueous nitric acid solution)for one to ten hours at 60 to 90° C.

To the resultant carbon fiber dispersion, an aqueous solution ofchloroplatinic acid is added, thoroughly stirred and, then, an excessamount of a reducing agent such as formaldehyde is added to allow areaction to proceed and, thereafter, a solid article is recovered byfiltration. By drying the thus-recovered solid article in an atmosphereof an inert gas such as argon at 120 to 500° C., a catalyst carrier canbe obtained in which platinum fine particles are carried on thevapor-grown carbon fiber.

The catalyst carrier according to the present invention is a catalystcarrier in which fine catalyst metal particles are carried on avapor-grown carbon fiber as a carrier and has an improved catalystactivity per unit amount of the catalyst metal compared with a case inwhich a carrier in powder form such as carbon black is used. Further, byusing the vapor-grown carbon fiber subjected to a crushing treatment, agrain diameter of the catalyst metal particles to be carried comes to beapparently small compared with a case in which the crushing treatment isnot performed (namely, the specific surface area of the catalyst metalcomes to be large). Specifically, it is possible to allow an averagegrain diameter of the catalyst metal to be carried to be 15 nm or lessand, further, 10 nm or less, which enables to enhance the catalystactivity of the catalyst support and to obtain favorable characteristicsfor use in the electrode catalyst for the fuel cell.

The catalyst carrier according to the present invention can be appliedfor an electrode material, a membrane electrode assembly for a fuelbattery, a cell for a fuel battery and a fuel battery and these articlescan be produced by a known method.

The electrode material according to the present invention can beproduced by forming a catalyst layer containing the above-describedcatalyst carrier on an electrically conductive base material such ascarbon paper, carbon fiber woven fabric or carbon non-woven fabric.Formation of the catalyst layer can be performed by, for example,applying a slurry containing the catalyst carrier on an electricallyconductive base material and, then, drying the resultant base material.The membrane electrode assembly for the fuel cell according to thepresent invention can be produced by thermally press-adhering theabove-described electrode material comprising a gas diffusion layer anda catalyst layer on both surfaces of an electrolyte membrane. In a casein which the fuel cell is of a solid polymer type, the electrolytemembrane comprises a polymer material and, for example, aperfluorosulfonic acid type polymer can be used. A fuel cell can beprepared by sandwiching the membrane electrode assembly by separatorshaving electric conductivity and two or more layers of such cell unitsare laminated one on another, to thereby prepare a fuel cell stackhaving a high output. Further, in order to suppress a leakage of aninner gas, it is also possible to provide a gasket between theelectrolyte material and each of the separators.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will specifically be described withreference to representative examples; however, these examples are givenfor an illustrative purpose only and by no means limit the invention.

Further, a ratio of an average fiber length was determined by obtainingaverage fiber lengths before and after carbon fibers were subjected to acrushing treatment in a cross-sectional photograph of carbon fibers by atransmission electron microscope (TEM). Further, a specific surface areawas measured in accordance with a BET method which is an ordinarymeasuring method of the specific surface area by using a specificsurface area measuring apparatus “NOVA-1200” manufactured byYuasa-Ionics Co., Ltd.

Example 1

5 g of vapor-grown carbon fiber having an average outer diameter of 150nm, a specific surface area of 13 m²/g and an average fiber length of 10μm was loaded in a rotary crusher (stainless steel-made crushing blade,rotation rate: 25000 rpm) and subjected to a crushing treatment for 10minutes. Thereafter, a specific surface area and an average fiber lengthof the carbon fiber were measured and, as a result, the specific surfacearea and the average fiber length were 18 m²/g and 7.5 μm, respectively,which were 1.6 times and 0.75 times the original values, respectively.0.2 g of the thus-crushed vapor-grown carbon fiber was dispersed in 50ml of distilled water, 0.172 g of sodium carbonate was added and then,stirred with heat at 80° C. To the resultant dispersion, an aqueoussolution containing 0.135 g of chloroplatinic acid was added dropwise,stirred for two hours and, then, a 35% aqueous solution of formaldehydewas added dropwise. After stirred for one hour, the resultant solutionwas filtered and, then, a solid material was subjected to a dryingtreatment for two hours at 400° C. in an atmosphere of argon, to therebyobtain a catalyst carrier in which a platinum grain was carried on acarbon fiber. A transmission electron microscope (TEM) photographthereof is shown in FIG. 11.

Further, diameters of the platinum catalyst were measured by observingthe TEM photograph, to thereby obtain a distribution of the diameters.As a result, an average diameter of the platinum catalyst particles was8 nm.

Example 2

5 g of vapor-grown carbon fiber having an average outer diameter of 150nm, a specific surface area of 13 m²/g and an average fiber length of 10μm was loaded in a rotary crusher (stainless steel-made crushing blade,rotation rate: 25000 rpm) and subjected to a crushing treatment for 20minutes. Thereafter, a specific surface area and an average fiber lengthof the carbon fiber were measured and, as a result, the specific surfacearea and the average fiber length were 24 m²/g and 6.0 μm, respectively,which were 1.8 times and 0.6 times the original values, respectively.0.2 g of the thus-crushed vapor-grown carbon fiber was dispersed in 50ml of distilled water, 0.172 g of sodium carbonate was added and then,stirred with heat at 80° C. To the resultant dispersion, an aqueoussolution containing 0.135 g of chloroplatinic acid was added dropwise,stirred for two hours and then, a 35% aqueous solution of formaldehydewas added dropwise. After stirred for one hour, the resultant solutionwas filtered and, then, a solid material was subjected to a dryingtreatment for two hours at 400° C. in an atmosphere of argon, to therebyobtain a catalyst carrier in which platinum particles were carried on acarbon fiber. A TEM photograph thereof is shown in FIG. 12.

Further, diameters of the platinum catalyst were measured by observingthe TEM photograph, to thereby obtain a distribution of the diameters.As a result, an average diameter of the platinum catalyst particles was5 nm.

Example 3

5 g of vapor-grown carbon fiber having an average outer diameter of 150nm, a specific surface area of 13 m²/g and an average fiber length of 10μm was loaded in a rotary crusher (stainless steel-made crushing blade,rotation rate: 25000 rpm) and subjected to a crushing treatment for 10minutes. Thereafter, a specific surface area and an average fiber lengthof the carbon fiber were measured and, as a result, the specific surfacearea and the average fiber length were 18 m²/g and 7.5 μm, respectively,which were 1.6 times and 0.75 times the original values, respectively.0.2 g of the thus-crushed vapor-grown carbon fiber was, after beingheated in a 60% aqueous solution of nitric acid for five hours at 70°C., dispersed in 50 ml of distilled water, 0.172 g of sodium carbonatewas added and the dispersion was stirred with heat at 80° C. To theresultant dispersion an aqueous solution containing 0.135 g ofchloroplatinic acid was added dropwise, stirred for two hours and then,a 35% aqueous solution of formaldehyde was added dropwise. After stirredfor one hour, the resultant solution was filtered and, then, a solidmaterial was subjected to a drying treatment for two hours at 400° C. inan atmosphere of argon, to thereby obtain a catalyst carrier in whichplatinum particles were carried on a carbon fiber. A TEM photographthereof is shown in FIG. 13.

Further, diameters of the platinum catalyst were measured by observingthe TEM photograph, to thereby obtain a distribution of the diameters.As a result, an average diameter of the platinum catalyst particles was6 nm.

Example 4

5 g of vapor-grown carbon fiber having an average outer diameter of 150nm, a specific surface area of 13 m²/g and an average fiber length of 10μm was loaded in a rotary crusher (stainless steel-made crushing blade,rotation rate: 25000 rpm) and subjected to a crushing treatment for 20minutes. Thereafter, a specific surface area and an average fiber lengthof the carbon fiber were measured and, as a result, the specific surfacearea and the average fiber length were 24 m²/g and 6.0 μm, respectively,which were 1.8 times and 0.6 times the original values, respectively.0.2 g of the thus-crushed vapor-grown carbon fiber was, after beingheated in a 60% aqueous solution of nitric acid for five hours at 70°C., dispersed in 50 ml of distilled water, 0.172 g of sodium carbonatewas added and the dispersion was stirred with heat at 80° C. To theresultant dispersion an aqueous solution containing 0.135 g ofchloroplatinic acid was added dropwise, stirred for two hours and then,a 35% aqueous solution of formaldehyde was added dropwise. After stirredfor one hour, the resultant solution was filtered and, then, a solidmaterial was subjected to a drying treatment for two hours at 400° C. inan atmosphere of argon, to thereby obtain a catalyst carrier in whichplatinum particles were carried on a carbon fiber. A TEM photographthereof is shown in FIG. 14.

Further, diameters of the platinum catalyst were measured by observingthe TEM photograph, to thereby obtain a distribution of the diameters.As a result, an average diameter of the platinum catalyst particles was3 nm.

Comparative Example 1

Same operations were performed as in Example 1 except that the crushingtreatment was not performed, to thereby obtain a catalyst carrier inwhich platinum particles were carried on the carbon fiber. A TEMphotograph thereof is shown in FIG. 15. Further, diameters of theplatinum catalyst were measured by observing the TEM photograph, tothereby obtain a distribution of the diameters. As a result, an averagediameter of the platinum catalyst particles was 19 nm.

Comparative Example 2

Same operations were performed as in Example 3 except that the crushingtreatment was not performed, to thereby obtain a catalyst carrier inwhich platinum particles were carried on the carbon fiber. A TEMphotograph thereof is shown in FIG. 16. Further, diameters of platinumcatalyst were measured by observing the TEM photograph, to therebyobtain a distribution of the diameters. As a result, an average diameterof the platinum catalyst particles was 23 nm.

Comparative Example 3

Carbon black available in the market (under the name of Vulcan XC-72Rmanufactured by Cabot Inc. having a specific surface area of 230 m²/g)was used as it was. 0.2 g thereof was, after being heated for five hoursat 70° C. in a 60% aqueous solution of nitric acid, dispersed in 50 mlof distilled water, sodium carbonate was added and the dispersion wasstirred with heat at 80° C. An aqueous solution containing 0.135 g ofchloroplatinic acid was added dropwise thereto, and the resultantdispersion was stirred for two hours and then, a 35% aqueous solution offormaldehyde was added dropwise. After stirred for one hour, theresultant solution was filtered and then, a solid material was subjectedto a drying treatment for two hours at 400° C. in an atmosphere ofargon, to thereby obtain a catalyst carrier in which platinum particleswere carried on carbon black. A TEM photograph thereof is shown in FIG.17.

Further, diameters of the platinum catalyst were measured by observingthe TEM photograph, to thereby obtain a distribution of the diameters.As a result, an average diameter of the platinum catalyst particles was2 nm.

TABLE 1 Treating conditions and powder characteristics Average Specificdiameter of Time Surface Hydrophilization platinum (min.) Area (m²/g)treatment particles (nm) Example 1 10 18 Not 8 performed Example 2 20 24Not 5 performed Example 3 10 18 Performed 6 Example 4 20 24 Performed 3Comparative 13 Not 19 Example 1 performed Comparative 13 Performed 23Example 2 Comparative 230 Performed 2 Example 3

Experiment Example Electrochemical Measurement

Each of the catalyst carriers prepared according to Examples 1 to 4 andComparative Examples 1 to 3 was mixed with a NAFION solution and theresultant mixture was applied on a carbon electrode, and anelectrochemical measurement was performed by using the resultantelectrode.

Activity per unit amount of a catalyst metal was evaluated from Tafelplots determined by an absolute value of a current density, alogarithmic number of the current density and a potential at a givenvoltage measured by a slow scan voltamogram (SSV). A 3-electrode typebattery was used in an experiment, wherein a glassy carbon electrode wasused as a working electrode, a platinum electrode as a counterelectrode, and a hydrogen electrode as a reference electrode.

As for a measurement condition, a current value was measured while avoltage was changed from 1.2 V to 0.4 V at a scanning rate of 1 mV/sec.In order to standardize the thus-measured current values to be a currentdensity (mA/cm²), a surface area of the electrode was obtained by usinga cyclic voltammetry. The measurement of the surface area of theelectrode was conducted with reference to a method as described inDenkikagaku Sokuteiho (Electrochemical Measuring Method) (Vol. I),Gihodo Shuppan Co., Ltd., p 88.

Results are shown in Table 2. The order of the absolute values of thecurrent density at a voltage of 0.5 V in Examples was as follows:Example 2>Example 1>Example 3=Example 4>Comparative Example1=Comparative Example 2>>Comparative Example 3.

Further, Tafel plots determined by a logarithmic number of currentdensity at a voltage of from 0.82 V to 0.94 V and the voltage are shownin FIG. 18. As is apparent from FIG. 18, it is found that, when any oneof the catalysts in Examples 1 to 4 was used in such a voltage range asdescribed above, a higher current density was able to be obtainedcompared with a case in which any one of the catalysts in ComparativeExamples was used and, accordingly, a catalyst performance has beenenhanced.

TABLE 2 Current density (mA/cm²) by SSV measurement −0.7 V −0.6 V −0.5 VExample 1 −0.2 −0.3 −0.4 Example 2 −0.3 −0.4 −0.5 Example 3 −0.2 −0.2−0.3 Example 4 −0.2 −0.2 −0.3 Comparative −0.15 −0.2 −0.2 Example 1Comparative −0.15 −0.2 −0.2 Example 2 Comparative −0.05 −0.06 −0.06Example 3

1. A catalyst carrier used for a fuel cell comprising a catalyst metal for promoting an oxidation-reduction reaction carried on a vapor-grown carbon fiber having a multi-layer graphene sheet structure, said vapor-grown carbon fiber comprising, at an end portion thereof, a discontinuous surface of a graphene sheet having a fracture surface and a closed portion having a continuous surface connecting an end portion of a graphene sheet with an end portion of an adjacent graphene sheet; and a hollow space in a center axis of the vapor-grown carbon fiber.
 2. The catalyst carrier as claimed in claim 1, wherein the discontinuous surface is configured in the direction of the circumference of the vapor-grown carbon fiber.
 3. The catalyst carrier as claimed in claim 1, wherein the discontinuous surface and the continuous surface are present at a same end of the vapor-grown carbon fiber.
 4. The catalyst carrier as claimed in claim 1, wherein the discontinuous surface and the continuous surface are present at different ends of the vapor-grown carbon fiber.
 5. The catalyst carrier as claimed in claim 1 having four or more graphene sheets, wherein outer two graphene sheets layers of the vapor-grown carbon fiber form a continuous surface over an entire outer circumference of an end portion of the vapor-grown fiber and inner two graphene sheets form both a discontinuous surface and a continuous surface at different inner circumferential positions of an end portion of the vapor-grown carbon fiber.
 6. The catalyst carrier as claimed in claim 1, wherein outer two graphene sheets of the vapor-grown carbon fiber form a continuous surface over an entire outer circumference of an end portion of the vapor-grown fiber and all other graphene sheets form a discontinuous surface at an end portion of the vapor-grown fiber.
 7. The catalyst carrier as claimed in claim 1, wherein an outermost graphene sheet and a graphene sheet fourth from the outermost layer and second and third graphene sheets from the outermost layer are combined with each other, respectively, to form a continuous surface over an entire outer circumference of an end portion of the vapor-grown fiber and all other graphene sheets form a discontinuous surface at an end portion of the vapor-grown fiber.
 8. The catalyst carrier as claimed in claim 1, wherein an outermost graphene sheet and a graphene sheet sixth from the outermost layer and second and fifth graphene sheets from the outermost layer are combined with each other, respectively, to form a continuous surface over an entire outer circumference of an end portion of the vapor-grown fiber and all other graphene sheets form a discontinuous surface at an end portion of the vapor-grown fiber.
 9. The catalyst carrier as claimed in claim 1, wherein the vapor-grown carbon fiber is entirely closed at one end thereof and has both a continuous surface and a discontinuous surface at the other end thereof.
 10. The catalyst carrier as claimed in claim 1, wherein the vapor-grown carbon fiber has each of a continuous surface and a discontinuous surface at both ends thereof.
 11. The catalyst carrier as claimed in claim 1, wherein the vapor-grown carbon fiber has an average outer diameter of from 15 nm to 500 nm, a BET specific surface area of from 4 m²/g to 100 m²/g and an aspect ratio of from 1 to
 200. 